It's quite expensive but SpinRite - http://www.grc.com/sr/spinrite.htm
is one of the few programs that can detect and try to fix any errors on a hard drive (long as it's not a physical/mechanical problem).
I would use Windows scan disk error checking software first and check the S.M.A.R.T data. If your talking about the $600 investment in your PC i'm sure it's fine unless your having an issue with that as well. It sounds like it's your external that is having the problems.